Fiber laser devices are known as one type of laser devices used in processing machines, medical devices and measurement devices. Laser light output from the fiber laser devices can have higher focusing performance, higher power density and smaller spot size as compared to laser light output from other laser devices. Furthermore, the fiber laser devices can perform precision work, process in a noncontact manner, and process a hard substance that absorbs laser light. Accordingly, applications of the fiber laser devices have been rapidly expanding particularly in the field of processing machines.
The fiber laser devices typically employ the MO-PA method in which laser light that is seed light having relatively low power is output from a master oscillator MO, the laser light is then amplified by an optical fiber power amplifier PA to a desired intensity and output therefrom. In the optical fiber power amplifier, seed light and pumping light are input to a rare earth-doped fiber, a rare earth element of the rare earth-doped fiber is pumped by the pumping light, and the seed light is amplified by stimulated emission of the rare earth element.
However, the fiber laser devices are disadvantageous in that the gain of the optical fiber power amplifier varies and the intensity of output laser light varies since the wavelength of pumping light shifts due to a temperature change of the use environment or a temperature change of the fiber laser devices caused by operation thereof.
Accordingly, Patent Document 1 discloses a fiber laser device that can suppress such variation in the gain to a small amount even when the wavelength of pumping light shifts due to a temperature change of the use environment. The fiber laser device disclosed in Patent Document 1 includes an even number of pumping light sources, half of which are configured to oscillate at a wavelength longer than a pumping wavelength at the peak of the gain of a rare earth-doped fiber and the other half of which are configured to oscillate at a wavelength shorter than the pumping wavelength at the peak of the gain of the rare earth-doped fiber. With such a configuration, even when the wavelength of pumping light shifts due to a temperature change of the use environment, the pumping light output from one half of the pumping light sources shifts toward the gain peak of the rare earth-doped fiber while the pumping light output from the other half thereof shifts away from the gain peak of the rare earth-doped fiber. In this manner, the gain variation of the rare earth-doped fiber is compensated by the wavelength shifts of the respective pumping lights to allow stable light amplification (Patent Document 1).